Until recently, cognitive science ignored the fact that most people of the world, and an increasing number of people in the US, are bilingual. In the past ten years this situation has changed markedly. There is now an appreciation that learning and using more than one language is a natural circumstance of cognition. Not only does research on bilingualism provide crucial evidence regarding the universality of cognitive principles, but it also provides an important tool for revealing constraints within the cognitive architecture. Although proficient bilinguals rarely make the error of speaking words in the wrong language or thinking that they are reading text in a language other than the one intended, recent cognitive research on lexical access in word recognition and in spoken production suggests that information about both languages is active, at least briefly, in even highly skilled tasks such as reading and speaking. The absence of a simple mechanism to switch off one of the two languages when using the other suggests that skilled bilinguals possess a sophisticated means of controlling their performance. For this reason, bilingualism has become an important tool for psychologists who wish to model developing systems, the competition between them, and the consequences for executive control. The goal of the proposed research is to use behavioral and neurocognitive methods to identify factors that permit cross-language competition to be resolved in the planning of speech in each of the bilingual's languages. The specifc aims of the planned research are to determine how far into speech planning there is activity of each language, how the form of bilingualism modulates this activity, and what contexts of language acquisition and use reduce cross-language competition. The proposed research will contribute important foundational knowledge about multilingualism that will inform educational and health issues in an increasingly diverse society in which many learners are faced with the task of learning to speak a second language past the earliest stages of childhood. The research will also contribute to the infrastructure of science by training a more diverse group of students than is typical in cognitive science and cognitive neuroscience and by fostering international scientific collaboration.